Interaction of the cyclic antimicrobial cationic peptide bactenecin with the outer and cytoplasmic membrane

Bactenecin, a 12-amino acid cationic antimicrobial peptide from bovine neutrophils, has two cysteine residues, which form one disulfide bond, making it a cyclic molecule. To study the importance of the disulfide bond, a linear derivative Bac2S was made and the reduced form (linear bactenecin) was also included in this study. Circular dichroism spectroscopy showed that bactenecin existed as a type I beta-turn structure regardless of its environment, while the reduced form and linear bactenecin adopted different conformations according to the lipophilicity of the environment. Bactenecin was more active against the Gram-negative wild type bacteria Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium than its linear derivative and reduced form, while all three peptides were equally active against the outer membrane barrier-defective mutants of the first two bacteria. Only the two linear peptides showed activity against the Gram-positive bacteria Staphylococcus epidermidis and Enterococcus facaelis. Bactenecin interacted well with the outer membrane and its higher affinity for E. coli UB1005 lipopolysaccharide and improved ability to permeabilize the outer membrane seemed to account for its better antimicrobial activity against Gram-negative bacteria. The interaction of bactenecin with the cytoplasmic membrane was determined by its ability to dissipate the membrane potential by using the fluorescence probe 3, 3-dipropylthiacarbocyanine and an outer membrane barrier-defective mutant E. coli DC2. It was shown that the linear derivative and reduced form were able to dissipate the membrane potential at much lower concentrations than bactenecin despite the similar minimal inhibitory concentrations of all three against this barrier-defective mutant.